1. Field of the Invention
The present invention relates to an optical disk device, and more particularly, to a technique for reproducing information recorded on an optical disk compliant with DVD-R and DVD-RW standards.
2. Description of Related Art
FIG. 16 shows a partial plan view of an optical disk compliant with DVD-R and DVD-RW (hereinafter, collectively referred to as “DVD-R/RW”) standards. Referring to FIG. 16, the optical disk has a groove area in which user information is recorded, and an area between adjacent grooves is called a land. Although the grooves are meandering, the grooves and lands are each shown in a rectangular shape for convenience of illustration.
Each land has auxiliary information, such as a physical address, recorded as a land prepit (LPP). In the case of recording information to be reproduced (hereinafter, referred to as “reproduction information”) on the optical disk, focused laser light is irradiated onto the disk surface, and the LPP is detected from a signal called a wobble signal which is obtained from the reflected light, thereby obtaining physical positional information of the optical disk. This makes it possible to record user information as an information mark at any position in the groove area. The meandering of each groove can be detected from the wobble signal, and a recording clock is generated from the wobble signal. Recording of the information mark in synchronization with the recording clock allows recording without causing any inconsistency even in the case of linking or overwriting data. Further, in the case of recording, laser power is increased and physical properties are changed by control of the temperature of a light focusing area, thereby forming the information mark.
During reproduction of the optical disk having the information mark recorded in the groove area, laser light is irradiated onto the groove, and a read signal containing reproduction information and synchronization information is obtained from the reflected light. During the reproduction, the laser light is oscillated with a smaller power than that required at the time of recording, to thereby prevent the information mark from being erased.
Note that the read signal obtained during the reproduction generally has a frequency higher than that of the wobble signal, and is called an RF signal.
FIG. 17 corresponds to FIG. 11 of Japanese Unexamined Patent Application Publication No. 2007-59018, and illustrates an example of a reproducing apparatus for reproducing an optical disk. The reproducing apparatus includes an A/D converter 11, an offset corrector 12, an asymmetry corrector 13, a PLL circuit 14, an error calculator 16, an optical head 17, an equalizer 19, a tap coefficient corrector 20, and a Viterbi decoder 21.
An optical disk 18 is rotated by a spindle motor (not shown) at a constant angular velocity or at a constant linear velocity. In the optical head 17, the distance between the disk surface and an objective lens and the radius positions of a groove and a focused light spot of the optical disk 18 are controlled by a servo circuit (not shown), and the focused light spot is irradiated onto the groove. The reflectance and polarization of the reflected light from the disk surface vary depending on the presence or absence of the information mark, and are detected by a detector (not shown) to thereby obtain an analog RF signal S1.
The A/D converter 11 performs A/D conversion on the analog RF signal S1, thereby obtaining a digitized RF signal S2. The offset corrector 12 performs offset correction for an output S2 of the A/D converter 11 based on an output of the error calculator 16. The asymmetry corrector 13 receives an output S3 of the offset corrector 12, detects asymmetry, and corrects the asymmetry. The PLL circuit 14 controls an output S4 of the asymmetry corrector 13 and a sampling clock of the A/D converter 11 to be synchronized in phase with the RF signal S1. There is also known a reproducing apparatus in which an AGC (not shown) performs a gain control for making the signal amplitude constant, before the output S4 of the asymmetry corrector 13 is supplied to the PLL circuit 14 and the equalizer 19.
The equalizer 19 equalizes the output S4 of the asymmetry corrector 13 by using a tap coefficient controlled by the tap coefficient corrector 20 so as to obtain a PR characteristic determined by the Viterbi decoder 21. When the SNR (signal-to-noise ratio) and channel characteristics of the input signal are substantially constant, the equalizer 19 may perform equalization using a fixed coefficient instead of using the tap coefficient from the tap coefficient corrector 20.
The Viterbi decoder 21 receives an output S5 of the equalizer 19 and performs maximum likelihood detection, for example, thereby obtaining detection data which is a channel bit data string.
The error calculator 16 receives the output S5 of the equalizer 19 and the detection data output from the Viterbi decoder 21, and outputs error information. Specifically, the error calculator 16 first converts the detection data into a signal (replica signal) expected as a corresponding input of the Viterbi decoder 21. Then, the error calculator 16 calculates a difference between the output S5 of the equalizer 19 and the replica signal, and outputs the difference only when the replica signal is located in the vicinity of the center of the reference level of the Viterbi decoder 21. The output serves as miscalculation information. The tap coefficient corrector 20 corrects the tap coefficient so that the difference between the output S5 of the equalizer 19 and the output of the error calculator 16 is minimized, and outputs the corrected tap coefficient to the equalizer 19.